1. Field of the Invention
The invention relates to a method of manufacture of a color television tube having low screen reflectance and to a tube obtained by means of this method.
2. Description of the Prior Art
The screen of a color television tube is mainly formed of vertical phosphor strips which are intended to emit light having respectively red, green and blue colors when they are excited by electron beams produced by the guns within the tube. These luminescent strips are not usually joined together but are separated by neutral strips, that is to say in which there is no phosphor material. The entire array of strips is coated with an aluminum layer which makes it possible in particular to reflect in the forward direction, that is to say toward the viewer, the light emitted in the backward direction by the phosphors. However, this aluminum layer constitutes in the neutral zones a mirror which reflects ambient light, thus impairing the quality and especially the contrast of the image. In order to overcome this disadvantage, an antireflection black layer formed especially of graphite is deposited between the phosphor strips prior to formation of the aluminum layer. This operation which consists in depositing a black layer in accordance with the invention is known as a "Matrix process".
The antireflection black layer is also deposited on the internal face of the flange or skirt of the faceplate (front portion of the tube on which the screen is formed) which joins said face-plate to the conical portion of the tube. In fact, reflections of ambient light from said flange could be a cause of disturbance for the viewer, especially when said flange is visible or in other words located outside the cabinet of the television receiver.
It has been observed that the known method of formation of the black antireflection layer has sometimes given unsatisfactory results since the deposit is not always uniform and can even exhibit blisters or cracks. Furthermore, this black layer is not of sufficient thickness.
The invention improves the known method in such a manner as to overcome this disadvantage. Provision is accordingly made for a uniform antireflection black layer of substantial thickness on the internal face of the face-plate. Furthermore, the method in accordance with the invention is inexpensive and can readily be employed for large-scale manufacture.
In order to gain a clear understanding of the invention and before describing the improvement, the known method of formation of antireflection layers will first be recalled.
The internal face of the front panel (faceplate and flange) of the tube is first washed and dried.
There is then applied on said internal face a photosensitive substance known as a photoresist which has the property of hardening when it is illuminated with ultraviolet (UV) radiation and of permitting removal by washing with water if it has not been illuminated. The photosensitive substance thus deposited is then dried, whereupon those zones of said substance which are placed at the future locations of the luminescent strips are then exposed to the UV radiations. Development with water is then carried out. Only hardened strips then remain on the internal face of the face-plate at the locations corresponding to subsequent deposition of the phosphor strips. Graphite in suspension in water is then applied on the internal face of the front panel. The strips of hardened photosensitive material, the glass which is in principle in the bare state between said strips as well as the face-plate flange are therefore all covered at the same time by the graphite just mentioned.
After drying, the next operation consists in spraying an oxidizing solution having a base of hydrogen peroxide H.sub.2 O.sub.2 with a view to producing a chemical attack on the hardened photosensitive substance which has remained on the glass beneath the graphite layer. Lastly, after a final development which consists in injecting water under pressure, the hardened photoresist which has been chemically attacked by the oxidizing solution is detached from the glass, thus carrying away the graphite coating. Only the graphite then remains on the face-plate and on the flange at the required locations, that is to say at the locations between the phosphor strips and on the flange.